MnSOD, a cellular defense factor against oxidative stress, also plays an important role in suppression of tumorigenecity. It is well documented that MnSOD activity is aberrantly reduced in a wide variety of transformed cells and overexpression of MnSOD suppresses cancer phenotypes. Importantly, MnSOD activity is inducible in cancer cells leading to protection of cancer cells against therapeutic agents. We have previously demonstrated that mutations in the MnSOD promoter region lead to a reduced expression of MnSOD in cancer cells and an NF-κB site in the second intron is responsible for cytokines-mediated MnSOD induction. Recently, we have found nucleophosmin (NPM), a RNA-binding nuclear protein, as a transcription co-factor of NF-κB in the induction of the MnSOD gene. Here, we investigate how the expression of MnSOD is regulated in cancer cells. Structural and functional studies indicate that formation of an RNA-like loop in the MnSOD promoter is negative for transcription factor complex to bind to the promoter-enhancer regions. Mutations carried in cancer cells interrupt formation of the loop structure resulting in a significant increase in the cytokine-mediated MnSOD induction. Co-transfection studies show that both NF-κB p65 binding to the enhancer region and Sp1 binding to the promoter region are necessary for the MnSOD induction. NPM enhances cytokine-mediated induction when the loop structure is not present. Chromatin Immunoprecipitation (ChIP) assay confirm that NF-κB-NPM-Sp1 complex binds to the enhancer-promoter regions in vivo. Two-hybrid and three-hybrid systems further establish aspects of protein-protein interaction and protein-DNA interaction in response to the transcriptional stimulation as shown in Figure. These results suggest that NPM functions as a novel mediator between promoter and enhancer to differentially modulate the expression of the human MnSOD gene under constituent and cytokine-induced conditions in cancer cells.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]